1. Field of the Invention
The present invention relates to an image sensor, and more particularly, a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same, in which light-shielding layers are formed in trenches to improve photosensitivity of the image sensor and simplify the process steps for manufacturing the CMOS image sensor.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is typically classified as a charge coupled device (CCD) image sensor or a CMOS image sensor.
The CMOS image sensor includes a photodiode area for sensing light and a CMOS logic circuit area for processing the sensed light to generate electrical signals. If amount of light received in the photodiode is great, the image sensor has excellent photosensitivity characteristics.
To enhance photosensitivity, a fill factor of an area occupied by the photodiode relative to the whole area of the image sensor can be increased. Alternatively, it is advantageous to change a path of incident light originally received by an area other than the photodiode so as to condense the light to the photodiode.
To condense light to the photodiode, a microlens is generally used. A convex microlens made of a material having good light transmittance is formed on the photodiode to refract incident light, thereby directing more light to the photodiode. In this case, light parallel to a light axis of the microlens is refracted by the microlens so that a focal point is formed on a certain position of the light axis.
Hereinafter, a conventional method for fabricating a CMOS image sensor will be described with reference to FIG. 1A to FIG. 1D.
As shown in FIG. 1A, a plurality of photodiodes 20 are formed in a surface of a semiconductor substrate 10 at constant intervals. Then, as shown in FIG. 1B, an interlayer dielectric layer 30 is formed on an entire surface of the semiconductor substrate 10, including the photodiodes 20, and a plurality of metal lines 32 are formed in the interlayer dielectric layer 30.
Next, an opaque metal layer is formed on the interlayer dielectric layer 30 and then selectively patterned by a photolithographic process to form light-shielding layers 40 that prevent light from entering an area other than the photodiodes 20. At this time, the light-shielding layers 40 are formed on portions corresponding to the metal lines 32.
As shown in FIG. 1C, a first planarization layer 50 is formed on the entire surface of the semiconductor substrate 10, including the light-shielding layers 40. Color filter layers 60 are formed on the first planarization layer 50 to correspond to the respective photodiodes 20.
The color filter layers 60 are comprised of red, green, and blue color filters. Each of the color filter layers 60 is formed in such a manner that a corresponding photoresist material is coated, and the photolithographic process is performed using a separate mask. The color filters are formed at constant intervals.
As shown in FIG. 1D, a second planarization layer 70 is formed on the entire surface of the semiconductor substrate 10, including the respective color filter layers 60. A material layer for microlenses is coated on the second planarization layer 70 and then patterned by exposing and developing processes to form a microlens pattern.
Finally, the microlens pattern undergoes a reflow process to form microlenses 80. The microlenses 80 condense incident light to the photodiodes 20 through the color filter layers 60 corresponding to the microlenses 80.
However, the conventional method for fabricating a CMOS image sensor has several problems.
When the light-shielding layers that prevent light from entering an area other than the photodiodes are conventionally formed by a patterning process, it is difficult to define a narrow area due to the resolution of the patterning process. If a narrow area is forcibly defined, problems such as particles may occur.
In addition, since the light-shielding layers are formed to be extended on the semiconductor substrate, the color filter layers are formed after the planarization layer is separately formed. In this case, the process steps are complicated, and the distance between the microlenses and the photodiodes increases, thereby deteriorating photosensitivity of the image sensor.